Re: The Wire Crossing a Toroid Winding Technique for Lowering Parallel Capacitance

On Oct 20, 11:54 am, D from BC <myrealaddr...@xxxxxxxxx> wrote:
On Sat, 20 Oct 2007 10:35:53 -0700, Paul Mathews <o...@xxxxxxxxxxx>
wrote:





On Oct 19, 1:46 pm, D from BC <myrealaddr...@xxxxxxxxx> wrote:
On Fri, 19 Oct 2007 12:50:47 GMT, "colin"

<colin.ro...@xxxxxxxxxxxxxxxxxx> wrote:

"Robert Baer" <robertb...@xxxxxxxxxxxx> wrote in message
news:13hgsjdj46dj768@xxxxxxxxxxxxxxxxxxxxx
D from BC wrote:

I'm aware of winding spacing and gap between first and last loops to
reduce toroid interwinding capacitance.

Then there's the winding technique where 1/2 the toroid is wound, then
the wire is pulled across the toroid to the beginning...and then the
windings resume. (one layer)

Normally with a continuous winding, the first and last loops meet and
this can be a heavy capacitance point due the Vdiff.

With the 'wire over the toroid' winding technique, 2 points of
greatest Vdiff are created. Each point with 1/2 the Vdiff compared to
the continuous winding. I recall a while ago somebody posted that the
'wire over the toroid'
technique can significantly lower inductor parallel capacitance.

I'm scratching my head on:

1) Does that 'wire over the toroid' have a name?
2) How does the 'wire over the toroid' technique reduce capacitance??
Is it the 2 points with 1/2 the Vdiff compared to normal winding?
Is it that the electric field gradient shape has changed from 'O' to
more like an '8'??

D from BC
Well, it does not make any difference where 2 wires come from, how they
are energized or even if they might be connected somehow elsewhere.
Two wires of a given length and spacing will have a given capacitance
(in that region) that can be easily calculated.

while that is in fact true, its effect on the total capacitance
depends on the turns ratio^2.

capacitance between adjacent turns has much less effect than from the
capacitance betweem turns at each end.

capacitance between the 'center tap' and either end has 1/4 the effect as
that from end to end.

Colin =^.^=

Ahhh...

Ok...I think I got this now....

Cp comes from the sum of interwinding capacitance, stray and terminals
too.
The interwinding behavior can be modeled as a LC array...
There can also be a LC array to a ground plane..
Cp is an extraction from those arrays.

Since Cp is derived from an LC array..wouldn't that make Cp voltage
and frequency dependant?

So..the higher the voltage ..the higher the Cp??

Now... for a fully wound toroid where the 1st loop meets the last
loop...Does that meeting point capacitance vary with voltage?
I don't think it should..
This part is structurally close to an ideal capacitor of which does
not vary depending on the voltage magnitude.

D from BC- Hide quoted text -

- Show quoted text -

The notions of lumped capacitance, resistance, and inductance, are all
useful. However, if you're working in switchmode, what you'd often
like to do is to maximize SRF. The higher the SRF, the less power loss
in snubbing, for example. Sometimes, we focus on some particular
parameter like leakage inductance, but it is really about minimizing
energy storage in parasitics, including capacitance. Having said that,
it's worth mentioning that you might find yourself actually going the
other way entirely with quasi-resonant designs.
Paul Mathews

Digital is sooo much easier..
With smps development, seems like everything jiggles like jello. :)

After some googling, I've noticed that commercial toroid inductor
makers seldom have the srf on datasheets.???
Now that tells me something..
Under the 'if you can't find it you don't need it' philosophy, it
means self resonant f is not important enough to put down on a
datasheet...
This makes me assume the off the shelf toroids have srfs high enough
to be manageable.
(I'm also a newbie with snubber circuits but have a fair
understanding.)
Or...it just means... if I want a particular inductance .it's going to
have it's associated Cp and srf and that's the way it is.. Tough! It's
up to other circuits to deal with it or control it.

Yeah... I've been thinking about resonant designs..
Quasi-resonant is like a 'why fight it...join it' approach.
I'll check it out.

D from BC- Hide quoted text -

- Show quoted text -

Real magnetics components depart widely from the ideal, and many are
poorly specified. Many circuit designers don't really know how to
apply them properly. Do some reverse engineering, and you'll find lots
of unnecessary shielding components, extra EMI filter stages, and
unnecessarily slow control loops out there in the real world, often
because designers don't really know what they're doing...they keep
adding components instead of solving basic problems such as high
energy parasitic oscillations. One more common example: quite often
magnetics are unpolarized, in the sense that you can plug them into a
circuit board in 1 of 2 orientations 180 degrees rotated. This can
make a tremendous difference in conducted EMI, since it can put the
outer turns of the component closer or farther away from a victim
circuit element. Details matter.
Lots of people in the magnetics business don't really know their trade
beyond getting UL recognition and making a component with nominal spec
sheet characteristics. Don't be surprised if their datasheets seem to
lack some really important information.
Paul Mathews

.

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